Traditional on-chip transmission lines may be embedded in a back-end-of-line (BEOL) region of a semiconductor package. As illustrated in FIG. 1, a conventional coplanar transmission line structure 10 is embedded in a BEOL region 12. The coplanar transmission line structure 10 includes a signal transmission line 14, a ground plane 16, shielding lines 18, and vias 20. The signal transmission line 14 and the shielding lines 18 are disposed on a same metallization level and the shielding lines 18 are electrically contiguous with the ground plane 16 through the vias 20. As such, the electric fields are formed horizontally between the signal transmission line 14 and the coplanar shielding lines 18. In addition, a sticking coating 22, which is used for metal deposition, is also formed in the BEOL region 12. The sticking coating 22 covers side walls and a bottom surface of each of the signal transmission line 14, the ground plane 16, and the shielding lines 18. Normally, the sticking coating 22 is formed of Tantalum Nitride (TaN). Furthermore, a top surface of each of the signal transmission line 14, the ground plane 16, and the shielding lines 18 are covered by an antireflective coating 24 to reduce optical reflection of these metals. Normally, the antireflective coating 24 is formed of Titanium Nitride (TiN).
It is clear to those skilled in the art that the TaN and TiN are poorly conductive materials. For low frequency applications, the current through the signal transmission line 14 will spread out the whole metal, such that the sticking coating 22 and the antireflective coating 24 may not obviously affect the quality (Q) factor and the insertion loss of the signal transmission line 14. However, for millimeter wave (mmWave) applications, the current through the signal transmission line 14 will circulate predominantly in these high resistivity coatings 22/24 at edges of the signal transmission line 14, the Q factor of the signal transmission line 14 will be poor, and the insertion loss of the signal transmission line 14 will be increased.
Accordingly, it would be desirable to improve designs of the transmission line structure in the BEOL region of a semiconductor package, so as to increase the Q factor and reduce the insertion loss of the transmission line structure for mmWave applications.